Drought and Heat Stress in Trees

Scorched tree leaves caused by drought or heat stress can sometimes be confused with diseases.

“Anthracnose on oak, maple, and ashes seems to be more commonly confused with scorching especially if the homeowner was not paying attention to the trees before the hot, dry weather. Anthracnose likes wet weather,” says Jim Schuster, Extension horticulture educator.

Some trees, he pointed out, respond to lack of adequate moisture and excess heat by shedding their leaves or leaflets when still green until the trees re-balance the foliage to available moisture.

“Other trees hang onto all their leaves but, then, the leaf tissue farthest from the veins dries out, turns brown, and dies,” he said. “The hotter and drier it stays, the more of the leaf tissue is killed. Scorching is the name given to the appearance of leaves that are partially alive due to drying out.”

Drought and drowning, interestingly, often cause similar appearances on the above-ground plant parts. When plants are too wet, the roots may suffocate or drown or the excessively wet soil may encourage root rot.

“As the roots die, the above-ground plant parts do not get enough water from the surviving roots,” Schuster said. “Therefore, the plants develop scorched foliage.

“When plants are in drought stress, there is just not enough moisture to keep all the foliage alive. Therefore, scorched leaves appear.”

To minimize scorching, Schuster said to make sure plants have adequate drainage. Plants growing on the side of a slope or hill do not necessarily have good drainage.

“Avoid over-watering, especially with an underground sprinkler system,” he said. “During drought, water deep but not frequently. Plants that are watered properly should be able to go a week or more between watering. Plants that are mulched correctly can go even longer between watering.

“For most trees and shrubs, try to water nine to 12 inches deep at least and deeper on sandy soils. Clay soils do not absorb water as quickly as sand or silt so water trees growing on clay at a slower rate to avoid or reduce runoff.”

On large trees, Schuster recommended starting watering several feet out from the trunk and water as far out as the tree is tall in all directions or as far as property lines, roads, and houses allow.